When an aircraft has landed and is on ground, the landing gear of said aircraft is subjected to ground loads. The landing gear comprises a trailing arm, connected to at least one of the aircraft wheels, and a shock absorber that dissipates most of the kinetic energy coming from the ground loads when the aircraft has touched the ground, the trailing arm usually being rotatable with respect to the shock absorber. The trailing arm of the landing gear usually comprises a pin bearing arrangement fixed to the trailing arm and located between the trailing arm and the shock absorber.
Aircraft landing gears usually comprise measuring elements to provide information for determining whether the aircraft is on ground or is still in flight.
In particular cases it can occur that, in some implementations of aircraft landing gears, the assembly formed by the trailing arm and the pin bearing arrangement does not rotate any angle up to a certain ground load level, when the aircraft has touched the ground. Thus, the measuring elements are not able to measure any rotation of the trailing arm, as it is inexistent, which makes that neither micro-switches nor proximity sensors can be used as valid measuring devices in landing gear pin bearings.
Pressure sensors that would be located inside the shock absorber cannot be used as valid measuring devices either because the internal mechanical structure construction of the shock absorber does not allow the correct installation of such pressure sensors.
According to what has been said, it shall be desirable to provide pin bearings with measuring devices that are able to reliably determine whether the aircraft is on ground or is still in flight.
The solution has then been found in the use of strain gauges located in the pin bearing arrangement. It is known in the state of the art, as per EP 1147384, the location of strain gauges in pin bearings, such that these gauges can detect a constant shear force applied in a certain region of the pin bearings. However, this document is silent about its use for indication of on-ground condition. Additionally, the correct location of these gauges is very complicated in practice, and in this document is focused on the use in testing ground loads.
One of the problems of the strain gauges in implementations on pin bearings of aircraft landing gears, where the assembly formed by the trailing arm and the pin bearing does not rotate any angle up to a certain ground load level, even when the aircraft has touched the ground earlier, is that these gauges have to be sensitive enough to detect a very low threshold load level in which the aircraft has already touched the ground. Thus, to work correctly in such cases, the gauges are to be located such that they can detect a lower load: the problem of this is that the gauges can give an indication that the aircraft is on flight, when it has already touched the ground.
However, if the gauges are located in the pin bearing arrangement in a location in which they can detect higher loads, they are then subjected to high stresses for a continued period of time, which originates strong fatigue problems in these gauges, therefore making the gauges have a short life. An additional problem is that the rotation of the trailing arm with respect to the shock absorber renders very variable the measuring conditions of the gauges in the pin.
It shall thus be desirable to provide a method for determining the positioning of the measuring elements in a pin bearing arrangements, as well as a method for providing an indication of the on-ground condition of an aircraft, using a pin bearing arrangement, in order to provide a suitable solution for the above-mentioned problems.